In these years, in automobiles, various measures are studied to improve fuel consumption in order to prevent atmospheric pollution and global warming. For automobiles to which measures to improve fuel consumption are applied, such vehicles are studied including idling-stop system vehicles (in the following, referred to as ISS vehicles) in which the operation time of the engine is reduced, and micro hybrid vehicles such as a generation control vehicle in which the rotation of the engine is used for power with no waste.
In the ISS vehicle, the number of times to start the engine is increased, and large current discharge is repeated in a lead acid battery every time when the engine is started. Moreover, in the ISS vehicle and the generation control vehicle, since the power output from an alternator is reduced and a lead acid battery is charged intermittently, the lead acid battery is often insufficiently charged.
The lead acid battery used in the manner described above is used in a partially charged state called the PSOC (Partial State Of Charge). When the lead acid battery is used under the PSOC, the lifetime of the lead acid battery is prone to be shorter than in the case where the lead acid battery is used in the full charge state. It is considered that the reason why the lifetime is shortened when the lead acid battery is used under the PSOC is that when charging and discharging is repeated in the state in which charge is short, lead sulfate generated on a negative plate in discharging becomes coarse and the lead sulfate is hard to return to metal lead which is charge product.
Moreover, in charging, sulfuric acid ions are generated from lead sulfate which is discharge product, and sulfuric acid ions are heavy. Thus, sulfuric acid ions tend to move below in electrolyte. In the typical use of the lead acid battery, the electrolyte is stirred due to gassing in the end of charging, and it is suppressed that sulfuric acid ions are moved below. However, since the battery is not fully charged under the PSOC, the electrolyte is hardly stirred due to gassing. As a result, concentrated electrolyte resides in the lower part of a battery container, dilute electrolyte resides in the upper part of the battery container, and layers are formed in the electrolyte. It is difficult to accept charging (it is difficult to advance a charging reaction) under the concentrated electrolyte, and the lifetime of the lead acid battery is shortened.
Schemes for suppressing the formation of layers in the electrolyte are disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2002-025602), Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2004-063152), and Patent Literature 3 (Japanese Patent Application Laid-Open Publication No. 2006-059576).
Patent Literature 1 discloses a lead acid battery including a plate group in which an expanded grid positive plate accommodated in a synthetic resin envelope separator and an expanded grid negative plate having a glass fiber mat attached to its surface, are alternately stacked. In Patent Literature 1, a problem is described in paragraph 0003 that hydrogen gas generated from the negative plate surface does not sufficiently contribute to stirring electrolyte and the electrolyte is prone to form layers in the electrolyte. Patent Literature 1 is directed to a lead acid battery that is charged to fully charged state in which hydrogen gas is generated from the negative plate surface.
Patent Literature 2 discloses a lead acid battery including a plate group in which a positive plate, a negative plate entirely covered with an envelope separator, and a planar separator with a glass mat are laminated in multiple layers as the planar separator is disposed between the positive and negative plates (FIG. 5 (C)). It is noted that as described in paragraph 0007 in Patent Literature 2, it is recognized in the invention that the diffusion of electrolyte between the positive plate and the negative plate is hampered due to the structure of the envelope separator, causing the formation of layers in the electrolyte depending on concentrated electrolyte in the lower part of the plate and dilute electrolyte in the upper part of the plate during use. Patent Literature 2 does not solve the prevention of forming layers in the electrolyte.
Patent Literature 3 discloses a flooded-type lead acid battery separator formed of a microporous sheet bonded to a glass mat with some adhesive in order that layers are not prone to be formed in electrolyte even under the environment in which it is difficult to stir and circulate the electrolyte. However, the invention described in Patent Literature 3 does not disclose how to use the separator in the relationship between a positive plate and a negative plate.
Moreover, Patent Literature 4 (Japanese Patent Application Laid-Open Publication No. Hei-08-045537) discloses a configuration in which a U-shaped glass mat is disposed between a positive plate and an envelope separator made of a microporous synthetic resin film enveloping the positive plate in order to prevent a short circuit due to grid deformation caused due to a corroded positive electrode grid.